Electro-mechanical transducer



April 6, 1965 E. v. CARLSON ELECTRO-MECHANICAL TRANSDUCER Original FiledMay 5, 1960 2 Sheets-Sheet 1 24 INVENTOR. 541% %MW ea Apg' il 6, 1 965 Ev. CARLSON 3,177,412

ELECTRO-MEGHANICAL TRANSDUCER I Original Filed May 5. 1960 2Sheets-Sheet 2 INVENTOR.

United States Patent C 3,177,412 ELECTRG-MECHANECAL TRANSDUCER Elmer V.Carlson, Prospect Heights, 111., assignor to industrial ResearchProducts, Inc., Franklin Park, lllh, a corporation of Delaware Originalapplication May 5, 1%6, Ser. No. 27,006, new ,Patent No. 3,111,563,dated Nov. 19, 1963. Divided and this application Oct. 10, 1960, Ser.No. 61,748 4 Claims. (Cl. 317173) This invention relates to anelectro-mechanical transducer of the type employing a magnetic armaturehaving a portion vibratable in an air gap in a steady magnetic circuit.It also relates to an electro acoustic transducer which employs theelectro-rnechanical transducer in conjunction with a diaphragm orstylus. In this type of transducer, a permanent magnet provides a steadyflux at an air or working gap. This flux is referred to herein as thesteady flux, and the flux conductors and magnet or magnets constitutethe steady flux circuit. There is a coil around the armature which, whenthe transducer is used as a receiver, is energized so as to induce aflow of flux along the armature, and when the transducer is used as amicrophone, flux flowing along the armature induces a potential in thecoil. Flux moving along the armature as a result of a signal current inthe coil, or as a result of vibration of the armature, is called hereinthe signal flux, and the flux conductors forming this flux circuit arecalled the signal flux circuit.

The principal object of this invention is to enclose the signal fluxcircuit substantially entirely within an encompassing steady fluxcircuit. A feature of this invention is the positioning of aflux-conductive case in the steady flux circuit so as to besubstantially the sole magnetic conductor in part of the steady fluxcircuit. Contrasting this object and feature with existing transducerdesign, in the latter, the electro-mechanical transducer is a completeassembly within a case. The case, whether magnetic or non-magnetic, isspaced from a complete electro-mechanical transducer therein. That is tosay, the steady flux circuit of present transducers utilizes fluxconductors which perform the flux-conductive function and little more.While the conductors of the steady flux circuit may be related to theconductors of the signal flux circuit, they are more often than notarranged to separate the flux fields from the two circuits rather thanto combine them. Separation of the two circuits is desirable. Thegeneral object of this invention may also be expressed in this way, toenclose the signal flux circuit completely within the steady fluxcircuit entirely within any fields created by the steady flux circuit.As will appear, applicants case at all points is at substantially acommon magnetic potential with the result that flux fields originatingoutside of the case will affect only the steady flux circuit and willnot penetrate to the signal flux circuit. Conversely, the signal fluxcircuit and fields generated by it will be confined within the case.

The second object of this invention is to engage the poles of one ormore magnets with the inside wall of a flux-conductive case in such amanner as to retain the desirable common magnetic potential throughoutthe case. A feature of this invention is a magnet stack consisting oftwo elongated, spaced magnets, transversely magnetized with their outerpoles of opposite polarity and in physical engagement with oppositeinside walls of the case.

Another object of this invention is to center substantially an armaturewithin the flux-conductive case so that the armature is at very nearlythe same steady magnetic potential as the case.

Another object of this invention is to utilize a freely vibratableE-shaped or U-shaped armature. A feature of the invention is themounting of the ends of the outer ice arms of an E-shaped armaturefixedly in the space between the two magnets so that the entire armaturemay vibrate freely and not merely the end of the center arm which isdisposed in an air or working gap between the magnets. This is to becontrasted with structures holding the E-shaped armature along its basewhich necessarily involves establishing a fixed relationship betweensuch a holding means and the magnet.

An ancillary object is to limit transverse movement of the E- orU-shaped armature to a point well within the range wherein movement ofthe armature will exceed the elastic limit of its material. If thearmature should move a greater distance, it may take a permanent setwith permanent injury to the transducer. Applicant employs one of twomeans to accomplish this object. One means holds the armature at a nodeof vibration. This leaves the armature free to vibrate, but if thetransducer is struck as by hitting a floor and the shock is sufiicientso that the mass of the armature would deflect it so as to exceed itselastic limit, usually near the fixed gap, the holding means at the nodeof vibration will provide the necessary support. An alternative means isthe positioning of stops on either side of the armature and supported bythe case at a distance such that deflection of the armature is heldwithin safe limits, although permitting free vibration in normaloperation.

These and such other objects of the invention as may hereinafter appearare attained in the embodiment of the invention hereinafter describedand shown in the drawings, wherein:

FIGURE 1 is a perspective view, partly cut away and partly exploded, ofapplicants electro-acoustic transducer with the coil shownschematically;

FIGURE 2 is a top plan view, partly cut away;

FIGURE 3 is a view taken on the line 3-3 of FIG- URE 2;

FIGURE 4 is an elevation of that side showing the coil and lid terminalposts;

FIGURE 5 is a view taken on the line 5'5 of FIG- URE 2; and

FIGURE 6 is a fragmentary view of the end of the coil assembly engagingthe central arm of the E-shaped armature at its node of vibration.

Continuing to refer to the drawings, the transducer motor consists of aninternal assembly and a flux-conductive case. The internal assemblyconsists of a magnetic stack and armature and coil. The magnetic stackcom prises a magnet 10, flux-conductive pole piece 12, a spacer of lowflux conductivity 14, an E-shaped armature 16 having side arms 18 and 20and a central vibratable arm 22, a second spacer of low fluxconductivity 24, a second fiux-conductive pole piece 26, and a secondmagnet 28. The magnets are transversely magnetized as indicated. Thepole pieces 12 and 26 are coextensive with the magnets 10 and 28. Forsimplicity in construction, the two spacers 14 and 24 may be made from asingle U-shaped member of low flux conductivity which is slipped overthe end of the arm 20 of the armature 16. Around the arm 18 is a secondpair of spacers 30 of low flux conductivity. A coil 32 surrounds thecenter arm 22 of the E-shaped armature and is spaced therefrom. Betweenthe working end of the central arm 22 and the pole pieces 12 and 26,there are no spacers, but an air gap in which the armature arm 22 isfree to vibrate. This is called the working gap. The numerals witharrowhead lead lines 122 and 124 identify the fixed magnetic gaps, andthe numeral 126 identifies the working gap.

The foregoing describes the complete motor assembly, which istemporarily held in assembled relationship by any suitable means such asrivets or adhesives. This motor assembly however, would be highlyinefficient because there is no good iiux conductor connecting the northpole of magnet to the south pole of magnet 28. This conductor issupplied by the case which consists of a bottom receptacle or cup 34made of material of high flux-conductivity with a peripheral shoulder 36around the inside of its side walls. Seated against this shoulder is aclosure or cup 38, also made of fluxconductive material. Referring toFIGURE 5, the peripheral wall 40 of the closure 38 is in good magneticconnection with the upper part of the side walls of the bottomreceptacle 34.

With the parts thus far described, one has a highly eflicientelectro-acoustic motor. The steady magnetic flux from the magnets 10 and28 is provided with an excellent flux-conductive path through theclosure 38 and the bottom cup 34, a flux path so uniform that the entirecase is substantially at a common potential. Whatever flux fields aredeveloped within the case by the movements of the armature or a currentflowing through the coil, will be contained by this flux-conductivecase, and similarly any stray fields impinging upon the case will beabsorbed there in the steady flux circuit and not penetrate to thearmature and coil. As is well understood, where the armature moves as aresult of mechanical action, flux will flow in one or the otherdirection down the central arm 22 of the armature 16 and around theoutside arms 20 and 18 through the fixed gap of the magnets to completethe circuit through one or the other of the pole pieces. The flux linesare generally indicated by the dash lines 153. Where the source ofenergy is current in the coil, reversal of direction of current in thecoil causes reversal of direction of the flux along the arm 22 and thecircuit is completed as before. Referring to FIGURES 1 and 5, theclosure 38 is centrally aper tured and a diaphragm 40 is mounted on theoutside of the closure 38. Centrally of the diaphragm 40 is an-. choreda link 42 which is also anchored on the central arm 22 of the armature16. Mounted over the diaphragm is a lid 44 which is preferably offlux-conductive material which has a central opening 46, shoulder 48,and an insert 50 having a small central opening 52. This lid 44 ispressed against the bottom receptacle 34 adjacent the closure 38. Thesteady magnetic potential of the lid 44 is close to the common potentialof the direct flux circuit of the bottom receptacle 34- and the closure38.

In speaking of the case in this invention, the lid 44 may in fact be thecase, but this is because the diaphragm 40 is located outside of theclosure 38. The diaphragm can be positioned inside the closure underwhich circumstances the lid 44 would not be used. Similarly, if a styluswere connected to the armature, it would occupy a position such as thatof the drive link 42.

The signal flux circuit, therefore, consists of the armature 16 and thepole pieces 12 and 26. The steady flux circuit consists of the magneticstack and the magnetic case. The two circuits intercept each other atthe fixed gaps 122 and 124 and the working gap 126. The entire fluxcircuit is within the steady flux circuit whose major conductor, thecase, both shields the signal flux circuit from fields external to thecase and restricts signal flux fields to the case.

When the diaphragm or stylus is added to the electromechanicaltransducer described, one has an electroacoustic transducer.

Describing other elements of the invention, the numeral 54, referring toFIGURE 3, identifies an inertance tube which opens into the back ormotor cavity 56 or the transducer, and also into the shallow frontcavity 58, see also FIGURE 2.

The coil of wire 32 extending approximately to the dotted lines 60 inFIGURE 4 is either wound on a bobbin or encased in at least a partialenclosing shroud having the general configuration of the dotted lines62, and having the recesses 64 and 66 provided to provide an upper andlower limit of movement of the arms 18 and 20 of the armature in theevent of shock. Lead wires of the entire armature.

73 and electrically extending the ends of the winding, may also haveinsulating shrouds 72, 74, 68 and 70 serving to space the coil withinthe housing and to insulate the lead wires 73 and 75 from the housings.Alternatively, the excess material 62 exterior to the winding 60 may beeliminated and the coil positioned by placing adhesive cement betweenthe coil 32, and housing 34 and/or the housing 33, the coil in thisconstruction not being in physical contact with any portion of thearmature 16.

When one of these microphones or receivers is dropped on a hard surfacefrom a distance of several feet or more, the vibrating components suchas the armature may be subjected to an acceleration of several thousandtimes that of gravity. Even when one is mounted in a complete device,such as a hearing aid, in the normal manner with compliant supports, ithas been found that the acceleration may exceed 2000 times gravity whenthe complete device is dropped a distance of five feet. Accelerations ofthis magnitude in the direction of motion of the armature make thepresent type transducer inoperative or seriously impair theirperformance.

To provide performance characteristics which are independent oftemperature and ageing effects, it is desirable that the armature 16 beso supported that it can expand and contract without constraint and thatany changes in magnet and pole piece position should not influence therelative position of the armature center arm 22 with respect to theoperating air gap. The vibration amplitude of the armature has a maximumat the working gap end of the center arm 22 at 126. The amplitudediminishes gradually and is very nearly zero where the center arm 22 isattached to the ear transverse arm 16, along line 76, see FIGURE 1. Thisamplitude minimum or node may be slightly within transverse member 16;that is, slightly back of the plane of the rear end of the coil. Beyondthis node, the displacement is normally opposite in direction andincreases towards the cornersof the E where the transverse member 16joins the sidemembers 20 and then diminishes rapidly to zero at thepoint of attachment of the members 20 to the spacers 14. This node is avirtua fulcrum and at this point it is possible to provide a supportmember which is quite rigid in a direction normal or vertical to thearmature without sensibly affecting the vibration of the armatureproviding the support member does not prevent the armature from rockingabout this fulcrum region. The support member 83 in FIGURE 3 performsthis function. It may be spot welded or otherwise attached to thetransverse armature member 16 near the dotted line near where the centermember 22 joins it, or at approximately the dotted line shown inFIGURE 1. The exact location of the node will depend on the shape of theparticular armature used, but in any event, the preferred arrangement isone in which the support is placed at or near a node but in a way thatpermits substantially normal vibratory motion In the embodiment shown inFIGURE 3, the member 83 is cemented to the rear end of the coil whileremovable positioning members are placed in the opening in the coil tocenter the arm 22 in the coil. When this cement is rigid, thepositioning members are removed and the coil and armature, together withthe pole pieces, magnets and the rest 'of the motor sub-assembly arecentered within the bottom receptacle or cup 34. Just prior toinsertion, adrop of cement 85 is added to the under and upper sides ofthe coil so that the support member 83 and the coil 32 are cemented tothe cup 34. The cement on the upper part of the coil 32 serves to holdit in position with respect to cup 38.

Further shock protection is provided by the notches 64 and 66. These areso dimensioned and positioned. with respect to the arms 18 and 20 thatthey do. not prevent motion between these arms and the bobbin wall innormal operation. If, however, a high stress is applied to the armatureby sudden deceleration or acceleration, the sides of the slots serve asbumpers or stops to prevent excess motion. For moderate shockrequirements, the shock member 83 is not required. The size of theopening or slots in the coil are so dimensioned that the armature isstopped before it exceeds its elastic limits. In the structure shownwhich is inch square, it is desirable to limit the displacement of anypart of the armature to approximately .005 inch.

An alternative means of supporting the armature 16 at its node ofvibration is shown in FIGURE 6 where the numeral 71 identifies a portionof a metallic or plastic end wall of the coil, corresponding to 62 inFIGURE 3. A flattened diamond-shaped opening is dimensioned to engagethe central arm 22 close to the line 76 of FIG- URE 1. Movement of thewall 71 is constrained by the inside wall of the case, or by beingaflixed to the coil, which is otherwise restrained, as by the cement 85.

Where the transducer is used as a microphone, ideally the magnets are ofidentically the same length because if the geometry and relationship ofall of the parts is perfect, there will be no flow of flux along thearmature. Where the transducer is used as a receiver, the generalpractice of hearing aid manufacturers is to operate the coil at aselected direct current level with the result that with exactly equalmagnets, there will be a continual flow of flux in one direction alongthe armature. The preferred method of establishing a bucking orcounter-flux so as to balance the steady flux in the armature in thisdesign is to make one magnet longer or stronger than the other by havingthinner spacers between one pole piece and the clamped armature ends soas to oppose this flux generated by a continuous current through thecoil and exactly neutralize it. In this case, spacers 14 and 30 arereplaced by two thin spacers on one side and two normal spacers on theother side of the armature.

Applicant has shown an E-shaped armature. A U- shaped armature may beused, in which case the total area of the armature in the fixed gap willapproximately equal the total area of the armature in the working gap.Where the E-shaped armature is used, the combined areas of the two endsof the arms 18 and 20 in the fixed gaps equals the total area of the arm22 in the working gap. For ideal operation, the total area of the bottomreceptacle 34 and the closure member 38 should be approximately the samein order to establish a uniform steady flux potential in the case as awhole.

The electro-acoustic transducer described has a very low sensitivity toexternal fields (approximately 1% of conventional types), when used as amicrophone, and generates very little external field when used as areceiver. For a given separation, this permits up to 40 decibels moregain without instability or permits mounting more closely together(within a few thousand-tbs of an inch) with a conventional amplifier.This is done without any auxiliary magnetic shield that requires veryprecious space. Even a moderate acoustical pressure amplification of 100times (40 decibels) in a typical hearing aid necessitates an electricalpower amplification of 100 million times (80 decibels) or more. As theaid is made more compact, the separation between the microphone andreceiver is necessarily reduced if they are mounted in the same housing.This is normally the practice in back-oft-he-ear aids. Since themagnetic coupling varies roughly inversely with the cube of theseparation, the magnetic coupling problem becomes acute. Magneticshields external to the magnetic circuits of the transducer and externalto the complete transducer reduce the coupling but require a prohibitiveamount of space, and also increase the magnet leakage field, therebyimpairing the performance. Even though positioned to minimize coupling,the relative positioning for minimum coupling becomes too critical andcannot be preserved in production or in use. The design shown hereinreduces the electrical power induced in a microphone by a receiver (bothof this construction) by a factor of nearly 10,000 (i.e., an inducedvoltage ratio of nearly compared to the commonly used types. This makesit possible to mount the two transducers very close together withoutexcessive magnetic coupling in some applications. It also results in amore stable performance of the aid since the relative microphone andreceiver orientations are not critical.

An important advantage of the use of the case as substantially the soleflux conductor of a major portion of the steady flux circuit is that oneeliminates at least two thicknesses of flux conductors in the assembly.This substantially decreases the volume of the case and importantly,narrows the case along the dimension which is at right angles to theplane of the diaphragm.

The lid or top cover 44, FIGURES 1 and 3, when made of magneticmaterial, performs two functions: That of an additional magnetic shieldand that of forming the major wall of the front acoustic cavity 58. Dueto its good contact with the upper cup 38 and the lower cup 34, thecover 44 is substantially at the common potential.

The transducer shown is .375 inch square and .185 inch thick, thesebeing the over-all outside dimensions. The parts are very small. Thearrangement of the conductors comprising the two flux circuits, magnetsand coil, is important, therefore, not only from the standpoint offunction, but also because of their providing great compactness.

This application is a division of US. patent application Serial No.27,006, filed May 5, 1960 by the applicant, which has become Us. LettersPatent No. 3,111,563, dated November 19, 1963 Having thus described hisinvention, what applicant claims is:

1. An electro-mechanical transducer comprising two permanent magnetshaving poles of opposite polarity facing each other and constituting amagnet stack, an air gap between the magnets, a flux conductorconnecting the outer poles of the magnets to each other, an armature,one portion of the armature being mounted between and in low fluxconductive relationship with the magnets, another portion beingvibratable in the air gap, and a portion of the armature connecting itsfixed and vibratable portions lying outside the magnet stack and a coilaround said connecting portion of the armature.

2. An electromechanical transducer comprising two permanent magnetshaving poles of opposite polarity facing each other, a spacer of lowflux conductivity positioned between portions of the magnets forming amagnet stack with an air gap therebetween, a flux conductor connectingthe outer poles of the magnets to each other, an armature, one portionof the armature being mounted between and in low flux-conductiverelationship with the mag nets and another portion vibratable in saidair gap, and a coil around the armature.

3. The electro-mechanical transducer of claim 1 together with a polepiece flux-conductively mounted on that pole of each magnet adjacent thegap.

4. The electromechanical transducer of claim 2 wherein the fluxconductor connecting the outer poles of the magnets clamps the magnetsand spacer in assembled relationship.

References Cited in the file of this patent UNITED STATES PATENTS2,678,360 Bellis May 11, 1954 2,848,579 Russell Aug. 19, 1958 2,916,700Daschke Dec. 8, 1959 3,076,062 Fener Jan. 29, 1963 FOREIGN PATENTS523,157 Italy Apr. 14, 1955 1,044,169 Germany Nov. 20, 1958

1. AN ELECTRO-MECHANICAL TRANSDUCER COMPRISING TWO PERMANENT MAGNETSHAVING POLES OF OPPOSITE POLARITY FACING EACH OTHER AND CONSTITUTING AMAGNET STACK, AN AIR GAP BETWEEN THE MAGNETS, A FLUX CONDUCTORCONNECTING THE OUTER POLES OF THE MAGNETS TO EACH OTHER, AN ARMATURE,ONE PORTION OF THE ARMATURE BEING MOUNTED BETWEEN AND IN LOW FLUXCONDUCTIVE RELATIONSHIP WITH THE MAGNETS, ANOTHER PORTION BEINGVIBRATABLE IN THE AIR GAP, AND A PORTION OF THE ARMATURE CONNECTING ITSFIXED AND VIBRATABLE PORTIONS LYING OUTSIDE THE MAGNET STACK AND A COILAROUND SAID CONNECTING PORTION OF THE ARMATURE.